1. Field of the Invention
The present invention relates to an image forming apparatus, e.g. a photocopier, a laser printer, and facsimile, a process cartridge, and an electrophotographic photoconductor used in such image forming apparatus or process cartridge. Specifically, the present invention relates to an electrophotographic photoconductor that is excellent in stability of image qualities, and mechanical resistance (abrasion resistance), and a process cartridge for an image forming apparatus using such photoconductor, and an image forming apparatus using such photoconductor.
2. Description of the Related Art
Recently, organic photoconductors have been widely used as an electrophotographic photoconductor (may also referred to as a “photoconductor” hereinafter). The organic photoconductors have many advantages over inorganic photoconductors, such that it is easy to develop materials of the organic photoconductors corresponding to various exposure light from various light sources, from visible light to infrared light, a material that has less possibility to pollute the environment can be selected to form an organic photoconductor, and a production cost of an organic photoconductor is inexpensive. One of the disadvantages of the organic photoconductors over inorganic photoconductors is poor mechanical resistance. In view of effective use of resources, the photoconductor preferably has a long service life.
An electrophotographic image forming apparatus is generally equipped with an electrophotographic photoconductor, a charging unit for charging the electrophotographic photoconductor, a latent image forming unit for forming a latent static image on a surface of the electrophotographic photoconductor which has been charged by the charging unit, a developing unit for depositing a toner onto the latent static image formed by the latent image forming unit, a transferring unit for transferring the deposited toner to a transferring medium, a cleaning unit for removing the toner remained on the surface of the photoconductor without being transferred, and the like in the integrated manner.
By repeating each process such as the aforementioned charging, developing, transferring, and cleaning, a surface of the organic photoconductor chemically or physically deteriorates, and therefore the abrasion thereof is accelerated, which may cause formations of scratches. As a result, image quality of resulting prints is degraded in the early stage. Therefore, improvement of the mechanical resistance of the organic photoconductor has been one of the most important tasks. To achieve this, various technologies for providing a protective layer have been disclosed for the purpose of improving the mechanical resistance of the organic photoconductor.
For example, many techniques, in which a protective layer is provided on an outermost surface of a photoconductor, and inorganic particles were dispersed in the protective layer for improving the mechanical resistance, have been disclosed. As one example of such techniques, an electrophotographic photoconductor, which is formed by at least a photosensitive layer, and a protective layer containing filler are sequentially formed on a conductive support, is proposed in Japanese Patent Application Laid-Open (JP-A) No. 2002-139859, and the like. Moreover, as another approach to tackle this problem, various techniques for improving mechanical resistance of a photoconductor by increasing a hardness of a surface of the photoconductor have been disclosed. For example, it is proposed in JP-A Nos. 2001-125286, and 2001-324857 that a hardness of a protective layer of a photoconductor be increased for preventing the photoconductor from being scratched by magnetic particles pressed against the photoconductor at a transferring section or cleaning section, in the case where a magnetic blush is used as a charging unit and the magnetic particles forming the magnetic blush are unintentionally transferred onto the photoconductor. Moreover, it is proposed in JP-A No. 2003-098708 that a hardness of a photoconductor be increased for preventing abrasions of a surface of the photoconductor in the case where a blade cleaning system is employed. As specific methods for increasing a surface hardness of the photoconductor such as those mentioned above, it is proposed that a crosslinkable material such as a thermoset resin, and UV-curing resin be used as a component for forming a protective layer of the photoconductor. For example, a method for improving mechanical resistance, and scratch resistance of a protective layer using a thermoset resin as a binder component of the protective layer is proposed in JP-A Nos. 05-181299, 2002-006526, and 2002-082465. Moreover, such technique is disclosed in JP-A Nos. 2000-284514, 2000-284515, and 2001-194813 that a siloxane resin to which a group capable of giving a charge-transporting ability is contained in a protective layer to improve mechanical resistance and scratch resistance of a photoconductor. It is also disclosed in Japanese Patent (JP-B) No. 3194392 that a charge-transporting layer is prepared using a monomer having a carbon double bond (C═C), a charge-transporting material having a carbon double bond (C═C), and a binder resin for improving mechanical resistance and scratch resistance. In JP-A No. 2004-302451, a method for forming a charge-transporting layer by curing a tri- or higher functional radical polymerizable monomer having no charge-transporting structure and a monofunctional radical polymerizable compound having a charge-transporting structure is disclosed. Moreover, in JP-A No. 2005-99688, a method for forming a protective layer is disclosed, and in this method a protective layer is formed by curing a tri- or higher functional radical polymerizable monomer having no charge-transporting structure and a radical polymerizable compound having a charge-transporting structure, and dispersing filler therein. According methods as mentioned above, mechanical resistance of photoconductors have been significantly improved. Especially, a photoconductor in which a curable resin disclosed in JP-A Nos. 2004-302451 and 2005-99688 is used in a protective layer has excellent mechanical resistance and scratch resistance.
However, it is difficult to achieve a long service life of a photoconductor just by improving mechanical resistance thereof. To attain a long service light of a photoconductor, it is important to prevent depositions of foreign matters, and improve toner transferring rate.
At first, the depositions of foreign matters will be discussed. Even a photoconductor having excellent mechanical resistance may produce defected images after use for a long period. The cause of the defected images may be depositions of paper powder, or additives of the toner. The parts of the photoconductor where these depositions are present are not properly charged or exposed to light, which may cause formations of defected images. The photoconductor having poor mechanical resistance can prevents generations of defected images, as an outermost surface thereof tends to be abraded, but it is difficult for such photoconductor of poor mechanical resistance to attain a long service life. Accordingly, it is very important to prevent depositions of foreign matters.
The toner transferring rate will be explained next. If the toner transferring rate increases, wasteful use of the toner is prevented. When an amount of the residual toner after transferring (the toner remained on the photoconductor without being transferred after transferring to paper being performed) is large, load to a cleaning unit increases. As a result, an effect of cleaning does not last long, which shorten a service light of a process cartridge. Therefore, it is very important to increase the transferring rate of the toner.
Since the similar properties are related to the prevention of the depositions of foreign matters, and improvement of a toner transferring rate, the both are described as a releasing ability of a photoconductor here. To apply the releasing ability, it is effective to lower the energy of the outermost surface of the photoconductor. As a method for lowering the surface energy of the surface of the photoconductor, there are an external addition method, i.e. a method for externally applying a material for reducing the surface energy, and an internal addition method, i.e. a method for adding a material for reducing the surface energy into the film of the photoconductor. As the external addition method, a system for applying generally zinc stearate or the like onto a surface of the photoconductor has been known. To employ this system, a releasing ability can be provided to the surface of the photoconductor. However, a surface of the material for reducing the surface energy is deteriorated by electric discharge, which may cause formations of defected images. Moreover, providing this coating system increases the size of the image forming unit, which reduces the freedom of layout. Furthermore, the cost of the image forming unit also increases. The internal addition method for adding the material for reducing the surface energy into the film is also effective for improving the releasing ability. A photoconductor having a surface layer that uses a fluorine-substituted polysiloxane resin for providing a high releasing ability to the surface of the photoconductor is disclosed in JP-A No. 2007-178815. However, it has been known that the siloxane bond causes polarization, and therefore hydrogen bonds may be formed. Therefore, the adhesion thereof to the toner tends to be large under high humidity environment. For this reason, the releasing ability reduces under the high humidity environment. Moreover, to localize the material for reducing the surface energy at the surface, it is necessary to always abrade the surface of the photoconductor. Therefore, the mechanical resistance of the photoconductor is compromised.
Achievement of both the mechanical resistance and releasing ability will be explained. To achieve both the mechanical resistance and the releasing ability, it is necessary to combine both the aforementioned method for improving the mechanical resistance and the aforementioned method for providing the releasing ability. However, the material used for releasing ability (repellency) tends to also affect other resin material phases coexisted, and therefore attaining both the mechanical resistance and the releasing ability is not easy to achieve.
There is disclosed in JP-A No. 2002-6526 a photoconductor including a protective layer in which lubricating particles are contained. Moreover, there is disclosed in JP-A No. 2008-139824 a photoconductor including a surface protective layer formed of a hardened product of a fluorine-containing hardening composition containing a fluoroalkyl group-containing (meth)acrylate and a photopolymerization initiator. Furthermore, there is disclosed in JP-A No. 2008-233893 a photoconductor including a protective layer that is prepared by hardening a fluoro UV hardening hard coating agent and a radical polymerizable compound having a monofunctional charge-transporting structure, and moreover that contains lubricating particles. Using a fluoromaterial is an effective method for reducing the adhesion between a photoconductor and a toner. Especially by adding such material into the hardened protective layer, both mechanical resistance and reduction in adhesion between a photoconductor and a toner can be attained. However, a large amount of the fluoromaterial needs to be added to sufficiently reduce the adhesion. Since such fluoromaterial does not have a charge-transporting structure, the addition thereof in a large amount may increase electric potential in a blight region in a latent electric image. In addition, the film strength tends to reduce.
Moreover, in JP-A No. 2003-302779, there is a description about a photoconductor including a surface layer formed of a resin component containing 100 parts by mass of a polyester resin binder, and 60 parts by mass of a compound having an acryloyl group in a norbornene ring, that is a specific example of a compound having an alicyclic carbon ring having carbon atoms of 7 or more and a polymerizable functional group, and a charge-transporting material. Moreover, in JP-A No. 2003-302779, other examples of the compound having an alicyclic carbon ring having carbon atoms of 7 or more and a polymerizable functional group are listed. In this case, the mechanical strength of the resin is low as the resin and the binder resin a large amount of which is used are not crosslinked. Therefore, this is not appropriate for extending a service life of a photoconductor.
As has been mentioned above, it is difficult to attain both mechanical resistance and a high releasing ability of a photoconductor, and it is the current situation that an electrophotographic photoconductor is designed to achieve either of the aforementioned properties.